Wind currently has an offer good until 31/8/2014 for a mobile broadband SIM at €14.90. It includes 30 days and 6GB data usage. (Their site says both 3 and 6 GB, but based on the SMS balance reply, it is 6 GB for me.)
http://www.wind.gr/en/for-individual...hout-contract/
Since I was used to the CosmOTE prepaid landing page, I was puzzled on how to activate it. I put it into my mobile broadband device and it started working. I wasn't sure what to do until I found the device received an SMS welcoming me. An SMS to 1228 with the command balance verified my 30 days had started.